Post-COVID-19 Syndrome Comprehensive Assessment: From Clinical Diagnosis to Imaging and Biochemical-Guided Diagnosis and Management

The COVID-19 outbreak was first reported in 2019, causing massive morbidity and mortality. The majority of the COVID-19 patients survived and developed Post-COVID-19 Syndrome (PC19S) of varying severity. Currently, the diagnosis of PC19S is achieved through history and symptomatology that cannot be explained by an alternative diagnosis. However, the heavy reliance on subjective reporting is prone to reporting errors. Besides, there is no unified diagnostic assessment tool to classify the clinical severity of patients. This leads to significant difficulties when managing patients in terms of public resource utilization, clinical progression monitorization and rehabilitation plan formulation. This narrative review aims to review current evidence of diagnosis based on triple assessment: clinical symptomatology, biochemical analysis and imaging evidence. Further assessment tools can be developed based on triple assessment to monitor patient’s clinical progression, prognosis and intervals of monitoring. It also highlights the high-risk features of patients for closer and earlier monitoring. Rehabilitation programs and related clinical trials are evaluated; however, most of them focus on cardiorespiratory fitness and psychiatric presentations such as anxiety and depression. Further research is required to establish an objective and comprehensive assessment tool to facilitate clinical management and rehabilitation plans.


Introduction
The COVID-19 outbreak was first reported in 2019, causing massive morbidity and mortality. Though various vaccine trials have shown good safety and efficacy for different populations, vaccination does not completely prevent infection and re-infection [1,2]. Though the majority of infected patients recover, a significant proportion develop Post-COVID-19 Syndrome (PC19S, aka Long-COVID-19 Condition) of varying severity. In October 2021, the WHO published the definition of Post-COVID-19 Syndrome (PC19S) as patients with a history of probable or confirmed SARS-CoV-2 infection 3 months from the onset of COVID-19 with at least 2 months of new-onset symptoms which cannot be explained by an alternative diagnosis. Currently, there are 12 documented domains of PC19S symptoms to define its diagnosis. However, the heavy reliance on subjective reporting is prone to reporting errors. Thus, an objective diagnostic tool set is required to define the diagnosis and facilitate future treatment.
Over 40-70% of COVID-19 survivors may develop PC19S [3,4]. This has a significant impact on their quality of life, mental health and employment. The effect of PC19S may last years or longer [5]. The impact of PC19S is enormous for patients, the healthcare system and economic development [6,7]. Multidisciplinary care aimed at identifying high-risk individuals and supporting long-term rehabilitation to maximize functional capacity is required [8]. Currently, the NHS has established a three-tier service model to provide targeted individualized interventions with a specific set of outcome measures outlined in

Current Diagnosis of PC19S
The WHO definition of PC19S is based on symptom duration, onset and category of symptoms. Figure 2 shows the most common PC19S symptoms. Various clinical questionnaires have been proposed with reference to the incidence of clinical presentations [14]. The initial questionnaire is used to screen the severity of PC19S. The change of symptoms score is used for patients with mild symptoms for outpatient monitoring [15,16]. The initial screening tool, "the long covid Symptom and Impact Tool", was constructed and validated in France, with a passing score of 30 for self-reported, acceptable symptom control state (95% CI, 28-33) [16]. Non-hospitalized patients are more likely to be associated with neuropsychiatric symptoms [17], while hospitalized patients have more respiratory symptoms (p < 0.05) [18]. Three months is set as the duration necessary for diagnosis, since over 70% of COVID-19 survivors experienced symptom resolution by 12 weeks [19].
Another validated clinical assessment tool in the UK is the "Post-Acute (Long) COVID-19 Quality of Life (PAC-19QoL) Instrument". It identified 35/44 quality of life indicators for those with PC19S (p < 0.05) [20]. Several other clinical tools are used for the assessment of PC19S, such as the Short-Form-36 Health Survey (SF-36), EQ-5D-5L Assessment Tool and Health-Related Quality of Life Scale (HRQoL) [21].
It may be difficult to accurately define PC19S as different assessment schemes emerge. Exertional intolerance and dyspnea with preserved or deteriorated lung function have both been reported [22]. It is important to divide the diagnostic/monitoring pathway into asymptomatic or symptomatic pathways first, followed by a triple assessment if available (clinical, radiological and biochemical). Table 1 shows an updated review summary of PC19S on various body systems.

Imaging Diagnosis of PC19S
Persistent lung injury has been reported in PC19S due to persistent shedding of the virus, the prothrombotic state and the ongoing inflammatory state [66]. Possible image findings include ground glass opacification, consolidation, crazy paving, vacuole sign, pulmonary nodules, lobar pneumonia, tractional bronchiectasis, vascular thickening, lung cavitations and fibrosis, predominantly at the peripheral/subpleural locations [67]. The CT severity score can be used to calculate the extent of pathological involvement in each of the 5 lobes [68,69]: Score of 0: no involvement; Score of 1: <5% involvement; Score of 2: 5-25% involvement; Score of 3: 26-50% involvement; Score of 4: 51-75% involvement; and Score of 5: >75% involvement.
The sum of each individual lobar score is added to give a global CT score to quantify the severity and predict the probability of PC19S [70]. A cut-off value of 7 is proposed with a sensitivity of 95.9%, specificity of 96%, positive predictive value of 95.92%, negative predictive value of 96% and accuracy of 95.96% [67].

Biochemical Diagnosis of PC19S
Understanding the inflammatory, immunological and metabolic activity biomarkers is required for the detailed assessment of PC19S [71].
Post-COVID postural tachycardia syndrome is one of the documented PC19S complications. Its diagnosis is based on clinical symptoms and an excessive orthostatic tachycardia (with a heart rate increase of >30 beats per minute in adults and 40 beats per minute in adolescents) within 10 min of upright posture in the absence of orthostatic hypotension and associated symptoms of orthostatic intolerance for at least 3 months [72,73].
In view of the various clinical manifestations of PC19S on the neurological system, some studies have investigated the effect of biomarkers on the central nervous system, such as myelin oligodendrocyte glycoprotein (MOG) and CCL-11 [74]. In patients with a neurological presentation of PC19S, reactive microglia with astrocyte reactivity, alteration of hippocampal neurogenesis, myelinated axons and oligodendrocytes have been reported in literature [75][76][77][78][79][80][81]. In view of this, several mechanisms have been proposed: (1) Cytokine-associated immune dysregulation disrupts myelin hemostasis and plasticity, impairing hippocampal neurogenesis with neurotoxic astrocyte reactivity. This results in an impairment of neural circuit function and cognition. (2) Anti-neural antibodies cause autoimmune encephalitis leading to immune-mediated brain injury (3) Neuroinvasive infection (4) COVID-19 triggers reactivation of latent virus infection, such as EBV, which can further trigger inflammation cascades and impairment of brain function (5) Neurovascular dysfunction of the blood-brain barrier leading to proinflammatory molecules and thrombosis. Neural inflammation accounts for the disseminated brain injury (6) Hypoxia and metabolic disturbances cause central nervous system injury Other biochemical research on PC19S have been in progress. Persistent clotting protein pathology has been reported with increased levels of antiplasmin [82]. Microclots with inflammatory molecules have been detected in plasma samples of patients with acute COVID-19 infection and PC19S. Consideration of anticlotting therapy to support fibrinolytic system dysfunction can be considered in clinically symptomatic survivors [31]. Anticoagulation can be effective in improving outcomes as persistent systemic vascular inflammation and dysfunction by thrombosis are key factors driving various complications in PC19S [83].
Several biomarkers have been identified as having prognostic significance. PC19S is associated with long-term inflammation, thus the persistence of interleukin-6 (IL-6) and tumor necrosis factor-alpha have been shown to be of prognostic value [84,85]. The magnitude of early CD4+ T cell immune responses correlate with the severity of initial infection; thus, it can be used as a tracker of PC19S severity [85]. Inflammatory dysbiosis of the oral microbiome is associated with the duration of symptoms of PC19S [86]. Thyroid dysfunction has been observed in PC19S patients. The presence of anti-thyroid peroxidase (Anti-TPO) is a positive prognostic finding for symptom resolution in earlier stage [87].
Detection of superantigens is a proposed hypothesis requiring validation. Persistent viral RNA shedding by superantigens could overstimulate the anti-virus immune response and induce negative feedback loops [88]. Immunologically, reduced CD4+ and CD8+ memory cells are associated with PC19S. Persistent T-cell perturbations up to several months are common in mild COVID-19 survivors [89]. Immunomodulation by mesenchymal cells could be a treatment option for these patients due to their immunomodulatory and regenerative properties [90].
Biochemically, an elevated taurine and reduced glutamine/glutamate ratio in plasma samples (p < 0.05) have been found in patients with PC19S, indicating possible liver and muscle damage and the related energy-demanding process of recovery and tissue repair [40]. Asymptomatic patients during follow-up display a spectrum of multiple biochemical patho-physiology. The metabolic phenotyping approach for multisystem functional assessment has been recommended [91].
The association between gut microbiota disruption and PC19S severity is under investigation [71].

Prognosis of PC19S Tools
Ongoing research on PC19S prognosis has aroused interest in the field. Table 2 shows a summary of useful investigation tools for patients with PC19S. A Bangladesh cohort study showed a persistence of PC19S symptoms at 9 months ranging from 16.1% to 50% [92,93]. The mean duration of PC19S symptoms was 21.8 weeks and the length of PC19S was projected with a structural equation based on age, gender, smoking and prior functional limitations [92]. Table 3 shows a summary of the clinical assessment of the prognosis and severity of PC19S in various studies in terms of triple assessment: clinical, biochemical and radiological.     Figure 3 shows the clinical follow-up schedule suggested according to the summary of existing evidence. Table 4 shows the persistence of the most common symptoms across different time periods after initial COVID-19 infection. The order of first ranked symptoms varies in different studies but the top 3 ranked problems are similar: ongoing respiratory problems, persistent fatigue and cognitive deficit (brain fog) [3,102,127]. The initial followup schedule should be set at 28 days, 3 months and 9 months. Further follow-up sessions can be arranged if there is a progression of symptoms or abnormal imaging or laboratory findings. Screening should be done based on clinical suspicion. COVID-19-associated acute kidney injury is associated with a good prognosis: over 70% can live without renal replacement therapy and over half with complete resolution of acute kidney injury [128]. Patients with persistence/progression of symptoms beyond 9 months should consider specialist referral as most symptoms disappear by 9 months. Triple assessment is recommended based on clinical, radiological and biochemical assessment for a comprehensive review. Rehabilitation programs are provided to patients, stratified by clinical severity to optimize resource usage and rehabilitation progress [8]. Hospital admission (odd ratio 2.28) and persistence of symptoms at day 28 (odd ratio 2.21) and month 9 (odd ratio 5.16) are independent predictors of suboptimal physical health and psychological stress [130]. However, another 9-month follow-up study showed no association between acute COVID-19 infection severity state and the number of PC19S symptoms and cognitive dysfunction [131].

Interval of Monitoring
There is no increase in inflammatory markers in blood tests at the subacute stage, and no association of the symptoms to biochemical parameters [132]. COVID-19-related persistent symptoms improve over time, but neurological symptoms last longer than other symptoms [98].
A 9-month grace period should be seen as a cut-off interval for top-up rehabilitation services since most PC19S symptoms should decrease after 9 months, while fatigue and somnipathy can persist for over a year [133].
PC19S has also been reported in non-hospitalized, asymptomatic patients [138]. However, continued interval monitoring for PC19S should be adopted before aggressive treatment since the longitudinal study in Faroe Islands showed symptom resolution took several months [139].
Another more recent study has shown that the number of initial symptoms is more related to PC19S than the severity of the acute infection [140].
Physicians in charge of pediatric populations should also look out for PC19S as it has been widely reported in various literature in children who demonstrate similar symptoms as adults, with an estimated incidence of over 40% [141][142][143]. Various clinical presentations have been documented such as myocarditis, postural tachycardia syndrome, mast cell disorder and mast cell activation [41, [144][145][146]. Patients with these complications should be screened and followed-up every 4 weeks or sooner for close monitoring.
PC19S is common among people living with HIV and moderate to severe acute COVID-19 illness. Over 20% reported PC19S at a median follow-up of 6 months, with the most prevalent symptoms being asthenia (80%), shortness of breath (50%) and recurrent headache (25%) [147]. Integrated assessment of PC19S and anti-retroviral therapy clinical visits should be considered [148].

Persistent and Clinically Significant Symptoms to Address
Most PC19S symptoms improve over time, particularly after 9 months. A significant proportion of patients reported persistent neuropsychiatric symptoms, which indicates a bad prognosis [133]. Brain fog has been reported frequently as a notable symptom of PC19S. There is an association of brain fog with abnormal FDG-PET findings, highlighted by a hypometabolic cingulate cortex [115]. FDG-PET imaging may also show hypometabolism in the parahippocampal gyrus, cerebellum, brainstem, olfactory gyrus, temporal lobe and thalamus, which corresponds to patients with persistent anosmia/ageusia, fatigue and vascular uptake (uncorrected p < 0.005) [149,150]. This corresponds to cerebral hypoxia, leading to compromised neuronal cell energy metabolism and, eventually, mitochondrial dysfunction [151]. The hypoxic condition favors persistent viral shedding and compromised cognitive change, which induces brain fog and related behavioral changes [151]. The UK Biobank MRI brain cohort study showed a greater reduction in grey matter thickness and tissue damage in the orbitofrontal cortex, parahippocampal gyrus and primary olfactory area at 4 months after infection, leading to multiple clinical presentations such as fatigue, olfactory dysfunction and brain fog [152]. Similar functional changes have been validated whole brain functional analyses performed at 11 months [103]. Another long-term follow-up study of dynamic brain changes in patients who recovered from COVID-19 without neurological manifestation showed there is a co-existence of recoverable and longterm unrecoverable changes after 10 months [153]. Cognitive decline in some patients may spontaneously improve over time [154].
Clinically, the assessment can be done using various scales such as the personality assessment inventory to assess somatic preoccupation and depression [116]. In patients with persistent neurocognitive symptoms, ANA titer elevations should be checked since autoimmunity could be a cofactor in the etiology of PC19S [155]. In patients with baseline motor dysfunction, PC19S with worsening motor symptoms requires a high dosage of levodopa for disease management [156].

Rehabilitation Program
The goals of rehabilitation should be individualized based on cardiorespiratory fitness, muscle function, bone and joint health, functional capacity and quality of life [157]. In terms of rehabilitation progress, clinically significant pulmonary function improvement may take a longer time, while muscle strength, walking capacity, sit-to-stand performance and quality of life usually improve earlier [158]. The initial lower physical composite score from the Short-Form Survey (SF-12) is associated with a prolonged recovery [99]. Figure 4 shows a suggested rehabilitation framework for patients with PC19S, and Table 5 shows clinicians' global research progress related to rehabilitation programs for PC19S.    Moderate impairment of physical capacity caused by muscle deconditioning has been observed in post-COVID-19 patients. Immobilization during its acute episodes further aggravates muscle deconditioning. Sarcopenia incidence is high, complicated by physical inactivity, lockdown, quarantine or acute hospitalization with bed rest [179]. Several smallscale studies have highlighted the importance of cardiopulmonary rehabilitation [180][181][182][183]. A recent clinical trial, the "COVID-Rehab Study", recruited post-COVID-19 survivors for rehabilitation. This study equips them with aerobic exercise, muscle strengthening and respiratory exercise 3 times per week over 8 weeks with the aim to measure the effect of exercise on various parameters in patients with PC19S, such as cardiorespiratory fitness, neuropsychological assessment, physical strength and functional capacity of body balance and inflammatory markers [167]. The use of oxygen-ozone autohemotherapy has been shown to induce a significant reduction in chronic fatigue in a trial of 100 patients (H = 148.4786, p < 0.01) [136]. The underlying reason for exercise rehabilitation for PC19S is related to the mediation of the anti-inflammatory response, the support of brain homeostasis and the increase in insulin sensitivity to counteract the neuropsychiatric and endocrine sequelae of PC19S [184].
Optimization of comorbidities is a key to rehabilitation. Diabetes mellitus and underlying cardiovascular risk factors play vital roles in disease management [149]. Strict control of diabetes with an optimization of comorbidities, supervised rehabilitation with physical exercise and an optimization of nutrition is recommended for the management of PC19S [185,186]. There is some evidence that the adoption of a plant-based diet could benefit PC19S management due to a reduction of pro-inflammatory mediators [187].
A 7-week psychologist-led interdisciplinary virtual rehabilitation program showed significant improvements in health-related quality of life in terms of mobility, self-care, activities of daily living, pain/discomfort and anxiety/depression [188]. Another 6-week interdisciplinary individualized pulmonary rehabilitation program showed promising findings including an improvement in the 6-min walking distance, pulmonary function test, post-COVID-19 functional status, Borg dyspnea scale, Fatigue Assessment Scale and quality of life [168]. Recent studies have also recommended the role of erectile dysfunction as a biomarker for systemic complications of PC19S, and rehabilitation of sexual health should also be included [189].
A multidisciplinary model showed effective improvement of self-reported symptom scores [190]. Primary care physicians are crucial to curb preventable adverse events through the use of clinical questionnaires, imaging assessments and serial biomarker monitoring to detect early complications and provide timely treatment [191].
In view of the growing number of PC19S cases globally, early identification of affected individuals to provide appropriate and efficient treatment is required [192]. The use of telemedicine for rehabilitation is suitable for long-term PC19S monitoring [193]. It provides a reliable and flexible assessment of patient's health status through remote monitoring, and identification of disorders and complications through long-term monitoring of health parameters. It also reduces patient's anxiety toward PC19S. Mental health video consultations is a useful alternative to screen patients with PC19S-related psychiatric symptoms in primary care [194]. Peer support groups are another trend which fills professional care gaps, raises societal awareness and increases public engagement [195].
Neuropsychiatric complaints are a major reason for consultation after 9 months. Several models have been proposed for these patients [196] such as the UT Southwestern Medical Centre COVIER Recover Program, UT Health San Antonio Program, VA Greater Los Angeles Healthcare System, Hennepin Healthcare and University of Florida Model. All of these are designed based on population characteristics, complication prevalence at different periods and resource availability. Baseline assessment for medical need is the first step in order to differentiate those who require self-management, acute hospitalization, community rehabilitation and more advanced rehabilitation based on their physical, cognitive and behavioral symptoms, and the relevant assessment scoring system. Multidisciplinary rehabilitation teams and specialist care will be provided for those with moderate/severe complications.
In view of long-lasting symptoms with a strong probability of progression in PC19S, various clinical studies focusing on symptom trajectory and rehabilitation progress have been published/are in progress. A brief summary has been documented in Table 5. Most of these studies primarily focus on cardiorespiratory fitness and neuropsychiatric symptoms such as chronic fatigue syndrome, depression, anxiety and functional independence measures. Yet, there is no unified rehabilitation program that has shown superior efficacy compared to others since individualized programs may be more efficacious than group training in terms of cardiorespiratory outcomes [82].
PC19S-related fatigue is a common symptom reported in literature. Rehabilitation has shown great efficacy in reducing fatigue. A systematic review and meta-analysis showed that over 80% patients reported no persistent fatigue after rehabilitation [197]. Several studies worldwide have shown that rehabilitation results in a statistically significant improvement in walking tests and the Borg Dyspnea Scale [166,[198][199][200][201][202].

Role of Vaccination in PC19S Rehabilitation
Vaccination confers partial protection from PC19S. An earlier study showed vaccination may contribute to a reduction in the population health burden of PC19S: the first dose reduces the odds of PC19S by 12.8% and the second dose reduces the odds by 8.8 (p < 0.01) [203]. However, in patients who already have PC19S, a delay of vaccination should be considered since a study showed that increases in antibody titers in patients with diagnosed PC19S might significantly worsen clinical symptoms [204].
The COVID-19 vaccine reduces PC19S sequelae by 15% (HR 0.85, 95% CI 0.82-0.89), including cardiovascular, hematological, gastrointestinal, mental health, musculoskeletal and neuropsychiatric sequelae [205]. Owing to the, at best partial, protection of PC19S resulting from COVID-19 vaccines, it is not an optimal strategy to reduce PC19S sequelae [205]. Vaccinated individuals are recommended to take effective infection control measures and booster vaccinations since vaccinated individuals reported less frequent and shorter PC19S symptoms in the British study [206].

Future Trends
In spite of the enormous population with PC19S, only 7% received PC19S care in the public sector and among those requiring additional help for PC19S, over half prefer assistance by digital health interventions (DHI) [207]. DHI-related research is under progress worldwide to understand its application in the remote monitoring of PC19S as it is seen as an economical rehabilitation model [208][209][210]. Future directions should include restructuring of PC19S rehabilitation services including both face-to-face consultations and digital interventions to allow flexibility.
The pathophysiology, prognostication scoring system and correlations with underlying comorbidities in different diseased populations are currently under investigation [211]. Mapping of clinical symptoms to the Human Phenotype Ontology for unification of PC19S reporting has been recommended to improve the stratification, diagnosis, treatment and scientific research of PC19S [212]. Different diagnostic criteria have been proposed, but further research into imaging and biochemical diagnosis is required for a definitive approach [213].
Multidisciplinary collaboration with a clear pathway for the diagnosis and management of PC19S is required globally in view of the surging number of patients with PC19S. A clear and integrated care system consisting of holistic healthcare pathways, detection of system-specific complications, management of mild symptoms and tailored rehabilitation services should be planned for in the future.